1. Field of the Invention
The present invention relates to a frequency output circuit for outputting a frequency signal, and, in particular, to a frequency output circuit comprising an open-collector output terminal.
2. Background Art
In order to reduce the emission of unwanted electromagnetic radiation from a frequency output circuit, the circuit should ideally output a sine wave. JP Patent Publication (Kokai) No. 2001-211047 A discloses that, in the case of a rectangular wave oscillator that is not of the open-collector output configuration, a rectangular wave is shaped to be closer to a sine wave using an inductor and a capacitor such that the unwanted electromagnetic radiation can be eliminated.
In frequency output circuits with an open-collector output terminal, the transistor on the output end is turned on or off and the signal voltage is received from an external power supply, such that the waveform that is outputted is basically a rectangular wave. When the output voltage occurs in the form of a rectangular wave, the current also flows through a rectangular wave. As a result, a high-frequency electromagnetic wave is emitted, which can have an influence on radios in terms of noise.
Therefore, in some known examples of the frequency output circuit with an open-collector output terminal, a current mirror circuit is provided whereby the current that flows through the open collector is limited.
Patent Document 1: JP Patent Publication (Kokai) No. 2001-211047 A